The invention relates generally to ultrasound transducers and, more particularly, to a method and apparatus for improving the acoustic performance of an ultrasound transducer by reducing artifacts within the acoustic spectrum.
Ultrasound transducers (i.e., ultrasound probes) have found application in medical imaging where an acoustic probe is held against a patient and the probe transmits and receives ultrasound waves. The received energy may, in turn, facilitate the imaging of the internal tissues of the patient. For example, transducers may be employed to image the heart of the patient. Increasingly, it has been desirable to minimize the size of ultrasound transducers to enable their use in intra-corporeal devices, such as trans-esophageal examination devices, laparoscopic examination devices, intra-cardiac examination devices, and the like. Such applications are quite demanding, requiring very small transducer packages that can nevertheless collect large amounts of information.
Ultrasound transducers typically have many acoustical stacks arranged in one dimension or in two-dimensional (2D) arrays. Each acoustical stack corresponds to an element within the transducer, and a transducer may have many acoustical stacks therein, such as several thousand arranged in the 2D array. To minimize space and electrical capacitance in an ultrasound transducer having a 2D acoustic array, it is preferred to join the acoustic elements directly to the electronics needed for transmit and receive beam-forming. A straightforward method for attaching 2D acoustic array elements to accompanying beam-forming electronics is to directly attach these two components using a conventional method such as solder balls, gold stud bumps, plated posts, etc. With this method of connection, however, acoustic energy from the array propagates into the electronics, leading to artifacts within the acoustic spectrum ultimately reducing the image quality of a medical diagnostic image. That is, as electronic components are typically made using silicon wafers, they possess a relatively low acoustic attenuation. Therefore, when an acoustic array (such as a 2D array) is directly attached to the silicon substrate, some of the acoustic energy generated on transmit will propagate into the silicon substrate. This acoustic energy will reverberate with minimal loss within the silicon substrate and can return to the acoustic array causing long ring-down and other acoustic artifacts. These artifacts reduce the quality of the acoustic image such as those useful in medical diagnostic imaging.
The acoustic artifacts can be reduced by placing a high acoustic impedance layer (i.e., a “dematching layer”) between the acoustic array and the silicon electronics. The use of such a dematching layer behind the acoustic array is well known to significantly reduce these artifacts by transforming the impedance of the layer on the reverse side of the dematching layer (i.e., the beam-forming electronics) and therefore increasing the impedance difference. However, the high impedance dematching layer itself does not reduce these artifacts sufficiently to provide the preferred image quality. That is, the ability of the dematching layer to reduce acoustic artifacts is less dramatic when the dematching layer is attached to silicon (such as the silicon substrate of the beam-forming electronics), as silicon itself possesses a fairly high acoustic impedance and low acoustic loss. An improved acoustic structure is thus required in order to provide for optimal acoustic imaging.
Therefore, it would be desirable to design an ultrasound transducer having an improved acoustic performance that reduces acoustic artifacts. It would further be desirable to maintain a minimal size for such an ultrasound transducer to enable its use as an intra-corporeal ultrasound probe.